Yes, Yahoo has broken themselves again with their latest upgrade, pretty, but technically useless for us. Their undocumented "shift-alt-f" worked until their last upgrade. In fact, for me (attempted on Firefox, Chrome, IE) their own "view full header" selection doesn't even work, so now you can do absolutely nothing to report spam.
I've sent them a support email that asks why every other large provider (Gmail, Outlook, AOL, etc.) has found a way to get full sourcecode in two clicks and why they make it so inconvenient/impossible to do the same, each and every single time they "upgrade" their email.
Since they've abolished working from home and have all their employees available for "better collaboration/innovation," there's no excuse not to see some results and get up to speed with industry standards.